Impact Dynamics Of Viscoelastic Droplets On Superhydrophobic Surfaces

The phenomenon of droplet impact on superhydrophobic surfaces is ubiquitous in industrial and agricultural production and daily life,such as rain droplets impinging lotus leaves,pesticide spraying,coating waterproofing,etc.Superhydrophobic surfaces with self-cleaning,anti-corrosion,anti-icing and other excellent hydrophobic characteristics often cause droplets to bounce,break and splash after collision,which is however not desired in specific scenarios where liquid-solid contact efficiency needs to be improved.Therefore,how to enhance the deposition of droplets on the superhydrophobic surface has become a problem worth breaking through.Viscoelastic droplets with shear thinning characteristics provide creative ideas and opportunities,which have great value for enhancing the droplet deposition effect.However,up to date,little attention has been paid on these issues.Therefore,this paper combines experiments and numerical simulations to study and analyze the impact of viscoelastic droplets on superhydrophobic surfaces.The main tasks and conclusions are as following:A superhydrophobic surface with a static contact angle of up to 160°was prepared by spraying method,and a total of 9 dilute solutions of PEO with different molecular weights(100000,600000,4000000)and different concentrations(1 g/L,5 g/L,10 g/L)were prepared.With the help of high-speed photography technology,the experiment of impact of water droplets and PEO polymer dilute solutions on superhydrophobic surfaces was carried out.Experimental results show that within the range of Weber number increasing from 0.08 to 120,diverse impact phenomena including deposition,complete rebound,partial rebound,splashing have been observed for water droplets and PEO-10W droplets.However,less impact phenomena occurred for PEO droplets with increasing molecular weights within our experimental parameters,e.g.,no splashing phenomenon was observed for PEO-60W droplets and only complete rebound and deposition were observed for PEO-400W droplets.As a result,increasing relative molecular mass and increasing concentration can inhibit the rebound of droplets on superhydrophobic surfaces,and narrow the range of Weber numbers for complete rebound.Further,increasing the relative molecular mass has a more significant effect on impact phenomena than increasing concentration.Since the inertial force determines the spreading process of the droplet,the maximum spreading coefficient₈(6)only depends on Weber number and has less dependence on the concentration of PEO and the relative molecular mass;with the increase of Weber number,the spreading time sharply decreases under low Weber numbers and then tends to be a constant,and the retraction time and contact time are reduced and then increased.Numerical simulation of PEO droplet impact on superhydrophobic surface is carried out with the help of CFD simulation technology.Using the phase field method,the simulation results agree with the experimental phenomena in the spreading stage.The simulation results show that the curvature of the central upper surface of the droplets increases when the droplets spread to a certain stage,causing the Laplace pressure to increase,and thus the vertical velocity of the central area of the droplets,forming a"collapse effect";in the impact process,the spreading coefficient is opposite to the law of the longitudinal stretching coefficient curve.The maximum spreading coefficient increases with the impact velocity,and the abscissa of the maximum value is offset to the left,which means that the time to reach the maximum spreading coefficient is shortened,and the maximum longitudinal stretching coefficient is positively correlated with the impact velocity.